When exposed to radiation such as X-rays, an energy-storable phosphor (such as stimulable phosphor which gives off stimulated emission) absorbs and stores a portion of the radiation energy. The phosphor then emits stimulated emission according to the level of the stored energy when exposed to electromagnetic wave such as visible or infrared light (i.e., stimulating light). A radiation image recording and reproducing method utilizing the energy-storable phosphor has been widely employed in practice. In that method, a radiation image storage panel, which is a sheet comprising the energy-storable phosphor, is used. The method comprises the steps of: exposing the storage panel to radiation having passed through an object or having radiated from an object, so that radiation image information of the object is temporarily recorded in the panel; sequentially scanning the storage panel with a stimulating light such as a laser beam to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as energy-storable phosphor sheet) used in the radiation image recording and reproducing method has a basic structure comprising a support and a phosphor layer provided thereon. However, if the phosphor layer is self-supporting, the support may be omitted. Further, a protective layer is normally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
Various kinds of phosphor layers are known and used. For example, a phosphor layer comprising a binder and an energy-storable phosphor dispersed therein is generally used, and a phosphor layer comprising agglomerate of an energy-storable phosphor without binder is also known. The latter layer can be formed by a gas phase-accumulation method or by a firing method. Further, still also known is a phosphor layer comprising energy-storable phosphor agglomerate impregnated with a polymer material.
The radiation image recording and reproducing method (or radiation image forming method) has various advantages as described above. Nevertheless, it is still desired that the radiation image storage panel used in the method have as high sensitivity as possible and, at the same time, give a reproduced radiation image of high quality (in regard to sharpness and graininess).
JP-B-4-75480 discloses a radiation image storage panel which comprises a phosphor layer containing a mixture of two kinds of stimulable phosphor particles having the same chemical composition but different sizes. The smaller particles have a size distribution in which the peak is positioned in the range of 1 to 8 μm, and the larger particles have another size distribution in which the peak is positioned in the range of 4 to 30 μm. (The difference between those peaks is at least 2 μm.) In the mixture, the weight ratio between the smaller particles and the larger particles is from 20:80 to 90:10.
JP-A-9-269400 discloses a radiation image storage panel comprising a phosphor layer in which the packing percentage of phosphor particles is 65% or more. The phosphor layer of the storage panel contains a mixture of smaller stimulable phosphor particles and larger phosphor particles. The smaller particles have a size distribution in which the peak (A) is positioned in the range of 0.5 to 5 μm, and the larger particles have another size distribution in which the peak (B) is positioned in the range of 6 to 30 μm. The weight ratio between the particles of the peak A and those of the peak B is from 5:95 to 40:60. The particles of the peak B are larger than those of the peak A by 5 μm or more, or otherwise are three times or more as large as those of the peak A.
JP-A-2000-284097 discloses a radiation image storage panel utilizing, as the stimulable phosphor particles, rare earth activated alkaline earth metal halide phosphor particles in the shape of tetradecahedron. The mean size (Dm) of the phosphor particles is from 3.5 to 7.5 μm, and the size distribution (Q) thereof is from 0.500 to 0.800. The publication also discloses that the phosphor particles having sizes of 5.0 to 8.0 μm and those having sizes of 2.0 to 4.0 μm are mixed in the ratio of 95:5 to 50:50.
JP-A-2001-124898 discloses a radiation image storage panel which comprises a phosphor layer containing stimulable phosphor and a light-reflecting layer containing light-reflecting material. The light-reflecting layer is provided on one side of the phosphor layer, and gives a scattering length of 5 μm or less when scattering a stimulating ray, which excites the stimulable phosphor.